Structural materials, functional materials, machine part materials, die materials, tool materials and other solid-state materials, such as glass, ceramics, diamond, cubic boron nitride (cBN) and tungsten carbide, are required to have an improved strength. To improve the strength means to prevent a chipping or cracking of a solid-state material when a force is applied to the solid-state material by a sporadic impact, repeated impacts or sliding.
In particular, high-hardness materials, such as diamond, binderless cBN sintered body and tungsten carbide, have a wear resistance and therefore are used for dies or edge tools such as cutting tools. However, these materials are brittle materials having low ductility and tend to crack, chip or otherwise break when an impact is applied thereto. Unlike metals, these nonmetal brittle materials are hardly plastically deformed. Therefore, when an impact is applied to these materials, the stress is concentrated at a small scratch in the surface formed during the manufacturing process to serve to extend the scratch. As a result, the scratch extends, and a cracking or chipping develops from the scratch.
A commonly known technique to improve the strength of a brittle material is to planarize the surface of the brittle material to remove a scratch or defect in the surface. Mechanical polishing using abrasive grain has been used for any material. Furthermore, Patent Literature 1 (Japanese Patent Application Laid Open No. 2007-230807) discloses a thermochemical polishing technique that removes micro cracks on a surface resulting from mechanical polishing as a technique for manufacturing a diamond product having high resistance to chipping.
Furthermore, a technique for improving the strength of glass is also a known technique for improving the strength of a brittle material. If a compressive stress is produced in the surface of glass, a scratch on the surface of the glass can be prevented from extending even if a force is applied to the scratch. Besides, a chemical reinforcement method (ion exchange method) is a glass reinforcing technique that involves immersing glass in a potassium nitrate (KNO3) solution to replace sodium ions (Na+) having a smaller diameter in the glass surface layer with potassium ions (K+) having a larger diameter, thereby producing a compressive stress in the glass surface (see Patent Literature 2 (Japanese Patent Application Laid Open No. 2011-256104)).
Furthermore, fiber-reinforced ceramics is also a known technique for improving the strength of a brittle material. For example, if several thousands or several tens of thousands of carbon or silicon carbide (SiC) fibers having a diameter of several μm to several tens of μm are bundled, although brittle fracture of each fiber occurs, brittle fracture of the fiber bundle is prevented because the fracture occurs in relatively small units. The fiber-reinforced ceramics is a composite material formed by binding a fabric of such fiber bundles with ceramics (see Patent Literature 3 (Japanese Patent Application Laid Open No. 2011-157251, for example)).
In the case where a surface of a solid-state material is planarized by mechanical polishing, scratches larger than the abrasive grain can be removed, but it is difficult to perfectly remove scratches formed by polishing with the abrasive grain. The thermochemical polishing technique disclosed in Patent Literature 1 takes advantage of the oxidation-reduction reaction between diamond and copper and cannot be applied to other solid-state materials than diamond. The techniques disclosed in Patent Literatures 2 and 3 are restricted in the solid-state materials to which the techniques can be applied.